1. Field of the Invention
The invention refers to an implantable medical system for treating a heart. In particular, the invention refers to an implantable heart stimulation system comprising an implantable heart stimulator such as an implantable pacemaker and/or an implantable cardioverter/defibrillator (ICD) and an electrode arrangement connected to the heart stimulator.
2. Description of the Related Art
Implantable heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. Strong enough a stimulation pulse outside a heart chamber's refractory period leads to excitation of the myocardium of that heart chamber, which in turn is followed by a contraction of the respective heart chamber.
Depending on the disorder to be treated, such heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator's housing and that carries a stimulation electrode in the region of its distal end. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse.
In order to be able to sense a contraction of a heart chamber, that naturally occurs without artificial stimulation and that is called intrinsic, the heart stimulator usually comprises at least one sensing stage that is connected to a sensing electrode on said electrode placed in the heart chamber. An intrinsic excitation of a heart chamber results in characteristic electrical potentials that can be picked up via the sensing electrode and that can be evaluated by the sensing stage in order to determine whether an intrinsic excitation—called: intrinsic event—has occurred.
Usually, a heart stimulator features separate stimulation generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event AP (atrial paced event) or a ventricular stimulation event VP (ventricular paced event), respectively.
Similarly, common heart stimulators feature separate sensing stages for each heart chamber to be of interest. In a dual chamber pacemaker usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events AS (atrial sensed event) or intrinsic ventricular events VS (ventricular sensed event), respectively.
As known in the art, separate sensing and pacing stages are provided for three-chamber (right atrium RA, right ventricle RV, left ventricle LV) or four-chamber (right atrium RA, left atrium LA, right ventricle RV, left ventricle LV) pacemakers or ICDs.
By means of a sensing stage for a heart chamber to be stimulated, the pacemaker is able to only trigger stimulation pulses when needed, that is when no intrinsic excitation of the heart chamber occurs in time. Such mode of pacing a heart chamber is called demand mode. In the demand mode the pacemaker schedules an atrial or a ventricular escape interval that causes triggering of an atrial or ventricular stimulation pulse when the escape interval times out. Otherwise, if an intrinsic atrial or ventricular event is detected prior to time out of the respective atrial or ventricular escape interval, triggering of the atrial or ventricular stimulation pulse is inhibited. Such intrinsic (natural, non-stimulated) excitation are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms.
In heart cycle, an excitation of the myocardium leads to depolarization of the myocardium that causes a contraction of the heart chamber. If the myocardium is fully depolarized it is unsusceptible for further excitation and thus refractory. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber is expanding again. In a typical electrogram (EGM) depolarization of the ventricle corresponds to a signal known as “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as “T-wave”. Atrial depolarization is manifested by a signal known as “P-wave”.
A natural contraction of a heart chamber thus can be detected by evaluating electrical signals sensed by the sensing channels. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a P-wave. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a R-wave. A P-wave or a R-wave thus leads to an atrial sense event As or a ventricular sense event Vs, respectively.
Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse is delivered to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other.
To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
A dual chamber pacemaker featuring an atrial and a ventricular sensing stage and an atrial and a ventricular stimulation pulse generator can be operated in a number of stimulation modes like VVI, wherein atrial sense events are ignored and no atrial stimulation pulses are generated, but only ventricular stimulation pulses are delivered in a demand mode, AAI, wherein ventricular sense events are ignored and no ventricular stimulation pulses are generated, but only atrial stimulation pulses are delivered in a demand mode, or DDD, wherein both, atrial and ventricular stimulation pulses are delivered in a demand mode. In such DDD mode of pacing, ventricular stimulation pulses can be generated in synchrony with sensed intrinsic atrial events and thus in synchrony with an intrinsic atrial rate, wherein a ventricular stimulation pulse is scheduled to follow an intrinsic atrial contraction after an appropriate atrioventricular delay (AV-delay; AVD), thereby maintaining the hemodynamic benefit of atrioventricular synchrony.
There are few approaches known to directly measure blood flow in the heart or a vessel close to the heart by means of an ultrasound Doppler sensor to be able to better adapt stimulation therapy to the needs of a patient.
Acoustic signals of various kind can be used for monitoring the cardiovascular and respiratory systems. Ultrasound in particular is widely used for both external and internal monitoring of cardiac structures and cardiac dynamics. Echocardiography especially is used extensively in non-invasive diagnosis and in the invasive form as Intra Cardiac Echocardiography (ICE). The standard external instruments are used for structural evaluations and also for dynamic functions such as blood flow and myocardial contractility measurements. Blood flow is usually measured by the Doppler effect and is extensively used in research work with more invasive techniques where catheters carry the ultrasonic crystal.
Echocardiography has become a powerful standard tool in the armamentaria of the cardiologist. It is used for many real time measurements such as blood flow measurement, heart valve timing observations and for many other purposes. These measurements are all performed from outside the body, e.g., by applying an ultrasonic transducer on the skin surface. Recently Intra Cardiac Echocardiography has become very popular. Intra Cardiac Echocardiography is carried out with catheters introduced into the heart. These catheters are highly specialized with some of them rotating at up to 1800 rpm and some having multiple crystals on them allowing full two dimensional cross sectional echo of the heart. This allows the close inspection of all the features of the heart and allow very precise location of catheters into the heart. In research various catheters probes and sensors have been used extensively. These are in the form of catheters with single or multiple crystals allowing such measurements as flow in small arteries and veins, in particular coronary arteries after a stent implant. These measurements are carried out using sonometry (distance measurement), Doppler for flow measurements and ‘time of flight’ for flow measurements. In sonometry and ‘time of flight’ measurement two ultrasonic crystals are required. These crystals are spaced apart. With sonometry the spacing needs to be the full span of the distance to be measured and in ‘time of flight’ the spacing is much shorter as only the influence on the speed of the ultrasound beam is measured.
The ultrasound measurements discussed use a range of frequencies from 1 MHz up to 40 MHz. The range of the measurement tends to dictate the frequency.
Although ultrasound is used extensively in acute measurements chronic measurements in humans have not been done effectively to date. Some chronic work has been done in animals but to a very limited extent. The reason for this is the complexity of the measurement and the power requirements. The complexity of the measurement entails the design of the probe, the positioning of the probe and the processing of the data. Ultrasound at higher frequencies can be made very directional and this is an advantage, but in an implant maintaining precise location over long periods of time is hard to do. In chronic human implants only very simple sensors have been realized and used sparingly. In the field of Pacing and Defibrillation only less than a hundred true sensors have been implanted to date as compared to the implanting of close to 500,000 devices per year.
There are numerous patents teaching the use of ultrasound in implantable devices. These patents range from the use of sonomicrometry to Doppler and in some cases even scavenging total acoustic energy.
The more relevant patents in this area are: U.S. Pat. Nos. 5,139,020; 5,156,154; 5,156,157; 5,183,040; 5,188,106; 5,243,976; 5,316,001; 5,334,222; 5,409,009; 5,544,656; 6,298,269; 6,398,734; 6,421,565; 6,757,563; 6,740,076; 6,795,732; 6,970,742; 7,037,266; US 2003/0083702; US 2003/0204140; US 2004/0176810; US 2005/0027323.